In auxiliary power units having a compressor and a turbine mechanically connected to one another and/or to an engine core, the compressor, turbine and engine core are typically sized to be able to accommodate peak loads. Accordingly, one or more components may not be operating efficiently during operation at average loads. Moreover, a compromise may need to be reached between the speed of the compressor and of the turbine, preventing one or both from operating at optimal speeds, which may compromise the aerodynamic efficiency of the auxiliary power unit.
Moreover, the compressor must typically be positioned in proximity of the component(s) to which they are mechanically connected, due to the geometric constraints of shafts, gearing, etc. This may limit the possible configurations for the auxiliary power unit, as well as the position of its main inlet, with which the compressor is fluidly connected.